1. Field of the Invention
The present invention relates generally to a system and process for wastewater treatment and, in particular, to a system and process for denitrification of wastewater.
2. Related Art
According to a recent article, which appeared in the Washington Post (“Troubled Waters in the Shenandoah: Death of Smallmouth Bass Brings Attention But No Quick Answers on Improving Quality” By Michael Alison Chandler, Washington Post Staff Writer, Wednesday, Jul. 20, 2005; Page B01), questions are constantly being raised about the quality of the water that feeds into waterways in and around the Shenandoah Valley and, ultimately, the Chesapeake Bay. Among the factors cited in the article, the high nutrient (and nitrogen) content in the feed waters was noted as a significant culprit:                The river is also known to have high nutrient levels. Nitrogen and phosphorus in high amounts cause excess plant or algae growth, which can reduce levels of dissolved oxygen. Fish struggle to breathe, and that can weaken their resistance to disease or bacteria.        The land along all three rivers affected by the fish kills is primarily agricultural. With more than 900 farms in the valley, the poultry industry dominates. High-nutrient waste from the farms is used as fertilizer and can wash into the river.It is clear that better, more effective ways to control the nitrogen content of waste water are needed.        
It is known, that the control of feed chemicals used in the processing of liquids (e.g., waste water in a waste water treatment system) can be automated through the use of computerized control devices. Problems can occur during the automatic dosing of chemicals into the treatment system because of inaccuracies of measurements of a chemical present in the system and the variable ratio of chemical to liquid when the liquid flow rate is variable.
In the past, dosing was done by laboratory or bench testing the influent chemical concentration, in combination with influent flow rate measurements. Subsequently, dose calculations were performed and the dosing device, a chemical feed pump, for example, was manually adjusted according to the calculations. More recently, partial pacing of the dosing pumps was practiced using an influent water flow signal. Varying the dose rate to maintain a desired effluent chemical concentration test result was deemed a more direct approach.
In recent years, reliable automatic analyzers for chemical concentration have become available, thus enabling automation of the entire dosing procedure. Automatic analyzers can also be set up to detect several important chemicals in waste water treatment system, enabling the use of a variety of chemicals depending on the specific application, e.g., the addition of sodium bicarbonate into an aerated biological reactor or the addition of iron or aluminum salts before a clarifier to control phosphorus removal.
In U.S. Pat. No. 6,129,104 (the '104 patent), the contents of which are incorporated herein by reference, a method for controlling the addition of liquid treatment chemicals by automatic dose control is provided. In this method, the calculation of the amount of chemical to be dosed into the system combines signals from a liquid flow meter, an influent chemical concentration analyzer, and an effluent chemical concentration analyzer. The signals are directed to a computerized chemical dose controller that analyzes and adjusts the data from the signals and generates an output signal that controls the chemical dosing mechanisms. According to the ‘104 patent, this method may be used, for example, for denitrification of wastewater using methanol as the feed chemical.
Denitrification comprises the removal of nitrate and nitrite from a waste stream through the use of facultative heterotrophic bacteria. These facultative heterotrophic bacteria, in the presence of a carbon source (e.g., methanol), and in the absence of dissolved oxygen (DO), can strip the oxygen atoms from both nitrate and nitrite moieties, leaving nitrogen gas (N2), which exits the waste stream and enters the atmosphere (air is about eighty percent nitrogen gas), hence “denitrifying” the waste stream. Thus, methanol consumption is dependent on influent nitrate and nitrite as well as influent DO, namely, enough methanol is required to first deplete the influent DO and subsequently to account for stripping all the oxygen atoms associated with nitrate and nitrite.
The ‘104 patent, however, ignores DO and nitrites and further describes measuring influent and effluent concentrations of nitrates only in order to determine an amount of methanol to be fed into the system for denitrification. However, as discussed above, the measurement of the influent and effluent concentrations of nitrates is insufficient in determining the proper amount of methanol to be fed into the system for denitrification. Stated another way a methanol dosing system that strictly looks at influent and effluent nitrate would not account for methanol demands associated with varying levels of influent nitrite and DO, thus leading to possible overdosing or underdosing of methanol. For example, relying on influent nitrate measurements only can result in overdosing because of a drop in influent DO levels. By the same token, reliance on effluent nitrate measurements can result in underdosing because a low level of measured effluent nitrate misses altogether the fact that a reduction in nitrate levels can simply mean that all the nitrate has been converted to nitrite, which must still be reduced. That is, nitrate is first converted to nitrite (thus leading to an increase in the levels of nitrite) on the way to complete conversion to gaseous nitrogen. Instead, the ‘104 patent relies on “fudge” factors, for instance, the use of an “adjustable factor [which] is determined by the operator” and/or the use of a “sensitivity factor [which] is selected by the operator” to compensate for the inaccuracy inherent in limiting measurements to nitrate concentrations. Initially these operator-controlled “factors” are no more than educated guesses and, at best, might be derived empirically. Implicit in such an operator-controlled technique is the necessity for an operator to “get up to speed” on system requirements, all of which represents a time consuming “learning curve.”
Therefore, there is a need for a more accurate, automated method of determining an amount of feed chemical (e.g., methanol) to be fed into a denitrification system without reliance on operator-controlled adjustable or sensitivity factors. In particular, there is a need for a method that takes influent concentrations of nitrogen-containing substances, in addition to nitrates (e.g., nitrites) (so called NOx), and dissolved oxygen and effluent concentrations of these nitrogen-containing substances (i.e., NOx) into account in calculating the proper dose of feed chemical. These measurements can be taken either from influent samples only or from both influent and effluent samples.